Tackling power conversion in auto electronics

As the electronic component count in automotive systems increases,
the amount of available space continues to shrink, greatly
increasing the density of each system. All of these systems require
power conversion ICs, usually with multiple voltage rails for each
subsystem. Traditionally, linear regulators provided the majority of
these power conversion needs as efficiency and small size were not
of significant importance.

But as the power density has
increased by orders of magnitude, and many applications require
relatively high ambient temperature operation, any practical
heat-sinking is too large to be accommodated. Thus power conversion
efficiency has become critical in order to minimize the power lost
as heat, driving step-down switching regulators to replace linear
regulators.

However emerging automotive designs require the
switching regulators to deliver very high efficiencies even with a
wide variation of supply voltages, very low quiescent current and
fast switching frequencies, all in a very compact, cost effective
solution footprint.

To maximize fuel mileage while minimizing carbon emissions,
alternative drive technologies continue to evolve. Whether these new
technologies incorporate an electric hybrid, clean diesel or a more
conventional combustion design, chances are they will also
incorporate a stop/start motor design. Already prevalent in
virtually all hybrid designs around the world, many European and
Asian and car manufacturers have also been incorporating stop-start
systems into conventional gas and diesel vehicles as well. In the
USA, Ford recently announced that it will incorporate such systems
into many of its 2012 domestic models.

Stop-Start Systems

Stop/start systems create yet another challenge for power management
systems. First, the battery must be capable of powering the vehicles
lights, environmental control and other electronics, while the
engine/alternator is off. Additionally, it must be capable of
powering the starter when the engine is once again re-started. This
extreme loading of the battery during start-up introduces yet
another design challenge, this time electrical, as the large draw of
current required to restart the engine can temporarily pull the
battery voltage as low as 4V, quite similar to the cold crank
voltage profile in Figure 1. The challenge for electronics arises in
supplying a well regulated output just a few hundred millivolts
below the input to keep critical systems running uninterrupted when
the battery bus voltage is briefly below its nominal 13.8V when the
charger returns to steady state.

Cold Crank“Cold Crank” is a condition that occurs when a car’s engine is
subjected to cold or freezing temperatures for a period of time. The
engine oil becomes extremely viscous and requires the starter motor
to deliver more torque, which in turn, draws more current from the
battery. This large current load can pull the battery/primary
bus voltage below 4.0V upon ignition, after which it typically
returns to a nominal 13.8V. The electrical result to the vehicles
power bus can be seen in Figure 1, but for different reasons. It is
imperative for some applications such as engine control, safety and
navigation systems to require a well regulated output voltage (at
least 3V) through a cold-crank scenario, so as to continually
operate while the vehicle starts.

You mentioned that no diodes are needed. I assume you were referring to flyback diodes since you have a synchronous output. However, automotive also has a reverse battery requirement which means adding a diode in the battery feed, unless the LT8610 has reverse voltage protection. This adds a diode drop at all voltages and means that the low end battery voltage is now 3.7Volts (assuming we can use a schottky diode). USB charging and data application require 5Volts. We really need a buck-boost supply.

You don't need an input diode for reverse battery protection. You can use a fet. This is a no-brainer if your low-battery-voltage operating-current is above half an amp or so.
Sepic seems easier to keep rf-clean. You have to use a boost controller rather than buck but with modern ceramic caps it's fairly plain sailing.